A lot of this is “freezer stash” — most bento hobbyists have staples than can be made ahead of time and frozen, for whenever you need a quick lunch, haven’t had time to go to the store, or just don’t have any inspiration that week. My freezer staples are bags of frozen dumplings, meatballs, and rice in both pre-molded onigiris and loose. Whenever I make rice, I put at least half a cup in a ziploc and stash it in the freezer. Defrost it, and it makes great fried rice.